This disclosure concerns optical wavelength filters for narrow bandwidth discrimination. An optical receiver is provided with a receiving lens assembly coupled to a birefringence and polarization optical wavelength filter having stacked retarders configured for tolerance to obliquely propagating light. The receiver is apt for a laser receiver operating in a diffuse medium, such as a short range undersea optical receiver responsive to modulated blue-green short wavelength laser communications. The receiver also is applicable to situations wherein a direct line of sight from the source to the receiver is uncertain and can be oblique to the optical signal path at the receiver.
A tunable multi-conjugate birefringence wavelength filter is disclosed in U.S. Pat. No. 6,992,809, which is incorporated in this disclosure in its entirety. In a filter as described therein, birefringent retarder plates are placed at predetermined rotational orientations, with associated polarizers at complementary orientations. The set of elements, comprising plural retarders and at least one polarizer, is configured such that certain wavelengths emerge at a polarization orientation that is aligned to the polarizer. Those wavelengths are transmitted. Wavelengths emerging orthogonal to the polarizer are blocked. Periodically related wavelengths emerge with the same orientation, and as a result, the filter transfer function is characteristically comb shaped.
Two or more filter stages as described can be placed sequentially, each comprising stacked retarders between polarizers at specific rotational orientations, to provide cascaded filter stages. The output or selection polarizer of one stage functions as the input polarizer determining the polarization alignment of the passband wavelength as transmitted to the next stage in cascade. According to one disclosed embodiment, at least one birefringent retarder plate stage is cascaded with a dielectric wavelength filter stage.
The transmission functions of cascaded stages multiply. If bandpass peaks in two cascaded stage transmission functions overlap, the result is a narrower bandpass peak and improved discrimination between the passband wavelength and out of band wavelengths. Where a bandpass peak in the transmission function of one stage corresponds to a stop band in the other stage, the bandpass peak is eliminated, adding to the free spectral range between other bandpass peaks.
For broadband light of a given polarization state, propagation through a birefringent retarder causes a change in polarization state that varies as a function of wavelength. Birefringence is an optical quality of certain crystals, such as calcite. Birefringent materials have orthogonal axes that have different optical indices. The two axes are sometimes termed the fast and slow axes. Effectively, the propagation speed of light is different for light waves that are aligned to one or the other of the respective birefringence axes.
An electromagnetic light wave likewise has orthogonal components. The relative amplitudes and the phase relationship between orthogonal light wave components define the polarization state of the light wave. It is possible that only one component may be present, such that the light is plane polarized parallel to that component. Similarly, two orthogonal components might be present at equal amplitude and in phase (or out of phase by an integer multiple of π radians), which corresponds to being plane polarized at 45°. There are various other possible arrangements including circular polarization (out of phase by π/2 radians), elliptical polarization (unequal amplitudes) etc. A plane polarized light signal can pass through a polarizer that is aligned to the polarization orientation of the light signal. A polarizer orthogonal to the plane polarized light signal will block transmission. Between these angles, a vector component parallel to the polarizer is transmitted and a vector component perpendicular to the polarizer is blocked.
When light wave components propagate through a birefringent material having distinctly different optical indices along orthogonal fast and slow axes as described, the vector components that are parallel to the fast axis propagate through the birefringent material more quickly than the components that are parallel to the slow axis. Therefore, propagation through the birefringent material alters the polarization state of the light wave by causing differential retardation. The amount of differential retardation varies as a function of the birefringence value and the thickness of the birefringent material. The retardation can be stated in units of time or propagation distance. Time and/or distance are variables that are independent of wavelength. However, a given propagation time or distance corresponds to a greater phase angle at a relatively shorter wavelength and a smaller phase angle at a relatively longer wavelength. The phase angle of the differential retardation is the pertinent parameter when considering a change in polarization alignment, not the differential time or distance. Thus, a differential retarder causes a change in polarization state that varies with wavelength.
A birefringent filter is configured so that a particular change in polarization state is achieved only in the particular wavelength bands to be discriminated by a selection polarizer. In cascaded stages, the selection polarizer of a given stage determines the input polarization state applied to the next stage. Each cascaded stage improves discrimination by narrowing the pass bandwidth (usually measured as the full width at half maximum or “FWHM”) and increasing the free spectral range (“FSR”) between peaks.
There are several standard configurations for stacked retarder birefringent filters that differ with respect to relative retarder plate thicknesses (e.g., equal thicknesses versus d:2d:d), rotational angles (e.g., rocking angles versus successively advanced angles), polarizer angles (parallel to input versus perpendicular). Examples are the Solc, Lyot and Evans configurations. Alternative arrangements are also possible.
Each tunable retarder in a stack generally comprises a tunable liquid crystal element paired with a fixed crystal retarder, wherein the fast and slow axes of the fixed and tunable retarders are aligned. Thus, increasing or decreasing the birefringence of the liquid crystal has the same effect as might be achieved by an thicker or thinner retarder, respectively. All the tunable paired retarders in a stack are tuned in unison. Therefore, if the filter configuration dictates retarders of equal birefringence, for example, (normally equal thicknesses), then tuning in unison increases or decreases the effective thickness of all the stacked retarders in the stack, maintaining the necessary relationship dictated for the filter configuration (equal thickness in this example) but changing the wavelength of the pass bands. Tuning to increase or decrease birefringence expands or contracts the comb filter transmission characteristic.
The stacked retarders comprise parallel plates. A light beam propagating along a central axis normal to the parallel planes of the plates passes through a thickness that is equal to the plate thickness. Similarly, collimated light from an image may be passed through the filter, whereby the light propagates along lines parallel to the central axis. In that case, the light beams pass along lines normal to the retarder plates.
A complication arises if the light from a source is directed through the filter along paths that are not parallel to the center axis, i.e., propagating along lines that are oblique to a line that is normal to the planes of the retarders. If light propagates along lines that diverge from a focal point, for example, the traversed thickness of a retarder plate is equal to the retarder thickness at a center axis, but is progressively thicker for light propagating at progressively more oblique angles. In the case of light from a wide field of view, incoming light beams that are parallel a line normal to the filter (in the center of the field of view) propagate through only the thickness of the plate. Light beams from the periphery of the field of view propagate diagonally through the plate, for example from the perimeter of the field of view, and thus should pass through a greater thickness of the retarders.
The transmission characteristics of the filter are a function of birefringence and thickness of the retarder plates. Therefore, propagation of different beams through different thicknesses produces different bandpass wavelengths over the field of view. It would be advantageous to provide a technique to obviate this problem so as to filter for the same or nearly the same bandpass wavelength over the field of view.